Events don’t necessarily always go as planned! Surprises can happen which will modify production forecasts. Good surprises: for example a field that continues in production beyond the time expected, 10 or 20% more oil volumes than were envisaged, it’s great for business! Or bad surprises: for example the productivity of a well can be inferior to forecasts. The quantity of oil produced each day can also be much lower than forecast, putting at risk the final profitability of the whole operation. The real disaster is when we are forced to prematurely stop production: an enormous investment almost entirely lost!
These surprises demonstrate clearly that the knowledge that can be gathered about the substratum is always uncertain, even if one has done everything possible to reduce this uncertainty. That is why, during the whole life of the field, re-evaluations are made on a regular basis of the reserves remaining to be extracted – new evaluations that can go in an upward or a downward direction. They are essential in order to know when the operation must be stopped.
The life of a field is variable: in general from 15 to 30 years. Longer for a really big one (a super-giant with a life of 50 years or more), shorter for deep-sea fields (5 to 10 years) because of the very high operating costs. The total life consists of 3 stages: the initial period (2 to 3 years) with a growth in extraction quantities as wells are sunk one after another. Then a long steady period during which annual production is stable. Finally a period of reduction, which varies from field to field, before the final closure.
All the oil and gas contained in the substratum is not extracted, far from it! Depending on the type of reservoir, recovery can vary from 10% to a little more than 50% for oil fields. For fields of gas alone, we can often reach 60 to 80%.
Therefore, significant quantities of oil remain in the substratum when operations are terminated. One of the challenges for the petroleum industry is to try to improve these recovery rates. Just a few percentage points gained on a major oil field represent enormous volumes. One percentage point of additional recovery in all the fields of the world would represent 2.5 to 3 years of annual worldwide consumption! |